The rapid advances in portable electronic devices such as cell phones, notebook personal computers, and so forth have been accompanied by increased demand for higher capacities from the batteries used in these devices as the main or back-up power source. The focus here is on non-aqueous electrolyte batteries such as lithium ion secondary batteries, which have higher energy densities than nickel/cadmium batteries and nickel/hydride batteries.
Typical examples of the electrolyte solutions in lithium ion secondary batteries are the non-aqueous electrolyte solutions obtained by dissolving an electrolyte, e.g., LiPF6, LiBF4, LiN(CF3SO2)2, or LiCF3(CF2)3SO3, in a solvent provided by mixing a high dielectric constant solvent, e.g., ethylene carbonate or propylene carbonate, with a low-viscosity solvent, e.g., dimethyl carbonate, diethyl carbonate, or ethyl methyl carbonate.
A carbonaceous material capable of the insertion/extraction of the lithium ion is primarily used as the negative electrode active material of lithium ion secondary batteries, and typical examples here are natural graphite, artificial graphite, amorphous carbon and so forth. Metal- and alloy-based negative electrodes that use, e.g., silicon or tin, are also known with the goal of further increasing the capacity. Transition metal composite oxides capable of the insertion/extraction of the lithium ion are mainly used as the positive electrode active material, and cobalt, nickel, manganese, iron, and so forth are typical examples of the transition metal.
It is known that the charge/discharge capacity is reduced by secondary reactions that occur between the electrodes and the electrolyte solution due to the fact that these lithium ion secondary batteries use a very active positive electrode and a very active negative electrode, and various investigations have been carried out on the non-aqueous solvent and the electrolyte in order to improve the battery characteristics.
An improvement in the battery characteristics is proposed in Patent Document 1 through the use of an electrolyte solution that incorporates an organic compound having at least two cyano groups. The battery characteristics are improved because the large dipole moment due to the polarity of the cyano group results in an inhibition of oxidative degradation of the electrolyte solution at the positive electrode during high-voltage charging.
Patent Documents 2 and 3 propose an improvement in the battery cycle characteristics through the addition of an isocyanate group-containing compound to the non-aqueous electrolyte solution, thereby inhibiting solvent degradation reactions.
Patent Document 4 proposes an improvement in the battery cycle characteristics through the addition to the non-aqueous electrolyte solution of an isocyanate compound and a prescribed sulfonic acid anhydride.
Patent Document 5 proposes an improvement in the battery characteristics by inhibiting LiPF6 degradation through the addition to the non-aqueous electrolyte solution of a phosphazene derivative.
Patent Document 6 proposes that the charge/discharge cycle life be improved by the formation of a passivation film on the surface of the carbon negative electrode; this is achieved by using a non-aqueous electrolyte solution that contains an alkynyl alkanesulfonate compound.
In Patent Document 7, it is reported that the cycle characteristics and rate characteristics are improved by using a silicon alloy negative electrode and an electrolyte solution that incorporates a diisocyanate and a fluorinated cyclic carbonate.
Patent Document 1: Japanese Patent Application Laid-open No. H 7-176322
Patent Document 2: Japanese Patent Application Laid-open No. 2005-259641
Patent Document 3: Japanese Patent Application Laid-open No. 2006-164759
Patent Document 4: Japanese Patent Application Laid-open No. 2010-225522
Patent Document 5: Japanese Patent Application Laid-open No. 2002-83628
Patent Document 6: Japanese Patent Application Laid-open No. 2000-195545
Patent Document 7: WO 2010/021236